A clear understanding of the manner in which the properties of thin films respond to changes in structure or composition is an essential prerequisite both to controlling their characteristics and to using them in practical applications. Examples of studies on epitaxial elemental semiconductors are described to illustrate how ambiguities arise in characterizing growth mechanism, degrees of structural order, and transport behavior. Some of the causes and consequences of disorder and nonstoichiometry are discussed with reference to wide band-gap semiconductor compounds.